Conventional printing inks, in particular letterpress and offset printing inks, are, in addition to other constituents, for example mineral oils, siccatives and anti-skinning agents etc., essentially composed of a varnish and pigments, there being used for four-colour printing, in addition to black, for which, for example, carbon black is used, organic pigments in the colours yellow, magenta and cyan. The varnish contains, for example, alkyd resins, hard resins and solvents or diluents.
The printing ink systems which are produced with the use of organic pigments, which is almost exclusively practiced nowadays, result in printing which is adequate in many respects on sheet-fed and rotary letterpress and offset printing, nevertheless there is a permanent requirement for further improvements, for example in colour strength, lack of hiding power (transparency) and brilliance as well as in the problems that derive from dispersions of particulate pigment bodies within the ink vehicle.
For example, with organic pigments these said properties cannot be improved beyond certain limits which are determined by, inter alia, the particle size of the dispersed pigments.
Dyestuff molecules have been immobilized in polyurethane matrices by chemical combination with the isocyanate reactant used to form the polyurethane. Typically the dyestuff was reacted with the isocyanate at the same time as the co-reactant used to form the polyurethane, typically a polyether or polyester polyol. U.S. Pat. No. 3,137,671 describes combining all the polyurethane forming reactants and the dyestuff simultaneously to obtain a fully cured polyurethane. It also describes premixing the dyestuff with some of the reactants before the final polyurethane forming reaction takes place. If the dyestuff is premixed with the isocyanate reactant a lower molecular weight adduct may be transiently formed. The finally cured high molecular weight polyurethane may be in the form of a foam or lacquer film or it may be ground and used as a pigment for colouring polyvinyl chloride molding resin. U.S. Pat. No. 3,228,780 describes simply adding a pigment with potentially isocyanate reactive amino groups to polyurethane forming reactants and obtaining a coloured foam. U.S. Pat. No. 3,994,835 discloses coloured polyurethane foams with chemically incorporated dyestuffs prepared by incorporating a dispersion of a dye with free amino or hydroxyl groups into the polyurethane foam forming reaction mixture.
In the present invention, printing inks, in particular letterpress and offset printing inks, are presented, which, compared with the conventional pigment-based printing inks, make possible considerable advantages in the lack of hiding power, the brilliance, the gloss of the printings or the depth of colour, and, where appropriate, offer advantages in several, or even all, of the said points.